Conventionally, as brake systems for vehicle (automobile), systems provided with a servo unit, such as a negative pressure booster or a hydraulic booster, are known. Further, in recent years, electric servo units using an electric motor as a boosting source are known (for example, see Patent Document 1).
The electric servo unit disclosed in Patent Document 1 includes a main piston that is moved forward and backward by operation of a brake pedal, a cylindrical booster piston that is fitted outside the main piston such as to be movable relative to the main piston, and a rotation-linear movement conversion mechanism, ball screws for example, for transmitting the rotational force of the electric motor to the booster piston as a booster thrust force.
By this electric servo unit, the main piston and the booster piston are used as pistons for a master cylinder, and the respective front ends are located in the pressure chamber of the master cylinder. Thus, an input thrust force applied to the main piston from a brake pedal by an operator and a booster thrust force applied from an electric motor to the booster piston can generate a hydraulic brake pressure in the master cylinder.